On all operating systems and devices, you can upload the video to Dropbox or Google Drive, then open the Dropbox or Google Drive on your phone and save it to your photos. Alternatively, you can email the video to yourself (if it's not too big!) and save it from there.

Posting to Instagram Stories

Posting to Instagram Feed

Make sure your video is 60 seconds or less, otherwise Instagram will make you trim it!

Third-party scheduling tools

There are a number of social media scheduling tools that make it easier to share to Instagram. Many can share directly to the Instagram feed, though there is still some manual work required to share to an Instagram Story.